During or after the manufacture of an article, defects that are difficult to detect based on a visual inspection of the article's exterior may occur. In some cases, the defects are present under an exterior surface of the article, such as under a surface coating, for example. Subsurface defects and other types of hard-to-observe flaws may have a number of undesirable consequences. In space systems, for example, subsurface defects may degrade thermal control surfaces due to comprised paint, increase the likelihood of electrostatic discharges (ESD) on satellites, diminish ESD mitigation on solar cells due to coating loss, permit contamination of optical components due to surface peeling and flaking, and cause rocket motor failure due to delamination of composite materials.
The advantages of non-destructive evaluation and testing (NDET) techniques to detect subsurface defects and other types of flaws are known. Thermography is one example of a NDET technique for detecting subsurface defects. Thermographic techniques generally involve subjecting a test article to a thermal pulse (e.g., from a flash bulb) followed immediately by an examination/evaluation of surface temperature differences using an infrared camera. One advantage of thermography is that it provides nearly instantaneous results. A possible disadvantage is that the thermal pulse may cause new defects if the test article contains volatile materials in micro-cracks, or if the composition of the test article includes materials with mismatched coefficients of thermal expansion. More benign NDET techniques are desirable.